Racing Suspension Setup - Complete Setup Guide, Part Two

In our first installment of this Setup guide, we explained the primary goals and all of the parts and pieces connected with the basic setup of your race car. We explained the relationship of the two suspension systems—front and rear—the mechanics of each, and how they need to work together. In this, Part 2, and in the final Part 3 next month, we'll examine the other areas of the race car design that could possibly affect and upset what we accomplished in Part 1.

2/12A few of the settings that can ruin an otherwise great setup are caster, camber, and bumpsteer. These three conditions affect the driver’s feel when steering the car through the turns as well as the amount of footprint and grip the tire will ultimately have. Let’s see how we can perfect those settings.

Even though we select the perfect combination of springs, moment center locations, and weight distribution for our car for the type of racing we do, it still might not be enough and seldom is. There are other influences on the handling of the car that need to be addressed and perfected before we can truly say we have a car capable of winning.

The geometry of our suspension systems aside from the moment centers is critical to how the car will roll around the track and how much grip will be available from our tires. There are five areas we will cover beyond Part 1 and those include: 1) caster and caster change, 2) camber and camber change, 3) bumpsteer, 4) toe settings and Ackermann, and 5) rearend alignment and rear steer.

It's very important to address each of these items in the order presented. Each can influence others and when we work in this order, we make sure not to get one thing ahead of any others that may affect the former step. For this part, we will cover the first three.

3/12Positive caster in the front wheel assembly is created when the ball joints are offset, from a side view, so that the upper ball joint is farther to the rear of the car than the bottom ball joint. The degree of caster is related to the angle in degrees that a line through the ball joints forms from a vertical line. In a stock car only turning left, we have caster split meaning there is less caster on the left side than the right side.

Caster Split and Caster Change
Caster is a design condition that, in addition to the spindle king pin angle, serves to cause a wheel to want to track straight ahead. A common example is a bicycle front wheel and fork assembly. The tube that the handlebars are mounted to is mounted so that the fork places the wheel axle and the point of the tire that touches the ground ahead of the part that rotates. From a side view, a line running from the top of the assembly to the tire contact point is angled, much like the spindle on a race car. If we turn the front wheel away from the direction of travel, it will want to return to straight ahead by the effect of caster. The same effect is present in the front wheel assemblies of our race car due to spindle caster inclination.

To ease the amount of effort it takes to turn the wheel in our race cars away from straight ahead, we introduce caster split into the design. Split means that we set different caster amounts into each front wheel assembly so that the car will want to turn to the left and thereby reduce the amount of effort it takes for the driver to hold the steering wheel when negotiating the turns. For dirt racing, we require less caster split, or in some cases, none at all.

4/12To measure caster, set the bubble at zero on the caster side of the gauge with the wheel turned to the left 20 degrees and then turn the wheel to 20 degrees in the opposite direction and read the scale. Some teams use one edge of the bubble so that reading the angle is more accurate than trying to estimate the center of the bubble.

Proper split for circle track racing means that the left front wheel will have less positive caster than the right front wheel. Positive caster is when the top ball joint is aligned to the rear of the bottom ball joint. In some cases, teams have been known to set negative caster in the LF wheel and positive caster in the RF wheel to affect the split.

To measure caster in each wheel, we use a caster/camber gauge. This tool attaches to the wheel hub. To check the amount of caster, we need to follow specific procedures. Refer to our illustrations for tips on how to do this correctly.

To adjust the amount of caster in each wheel, you'll need to move the upper ball joints, and in some cases the lower ball joint, fore or aft. To increase the amount of positive caster, move the top ball joint toward the rear of the car, or the bottom BJ toward the front. Some cars have slots cut into the upper chassis mounts for this purpose.

If you have permanently attached vertical mounting plates that the upper control arms are attached to, then you can vary the amount of shim spacing for each of the bolts that attach the control arm to the chassis. Wider spacing at the front bolt (control arm shaft inside of the mounting plate) will move the upper ball joint to the front creating less degree of caster at that wheel and so on. This is not the preferred method though.

5/12It’s best to use a digital gauge to measure your turn angles. Turn the wheels to the right from straight ahead so that they turn 20 degrees. Then level the gauge or push the “zero” button to set zero in the display. Then turn back to the left to 20 degrees past straight ahead and read the gauge for caster.

Once you have established the exact caster amounts for each wheel using the above method, (if not using slotted control arm shafts) you should order an upper control arm that has the ball joint offset to give the correct amount of caster at each wheel. That way, you can use the same shim spacing for each mounting bolt to connect the upper control arm shaft to the chassis.

Normal caster splits for most short track asphalt applications are around 2-4 degrees of difference. The LF caster might be 1-2 degrees and the RF caster might be 3-5 degrees. The higher the banking angle of the racetrack, the less caster split that is needed because less steering effort is needed due to the banking. The tighter the turn radius, the more caster split that is needed.

For dirt applications, smaller numbers are used because of the need to steer both left and right in many cases. Less overall caster angle and less split, or no split, is normal. A caster split as described above would require more than normal effort to turn the steering wheel right and would feel unnatural to the driver.

6/12Change the thickness of the adjustment shims at the front and back at each mounting bolt, or slide the control arm shaft, if it’s slotted, to adjust the amount of caster in each wheel. This will move the upper ball joint forward (to reduce caster) or to the rear (to add caster).

Camber and Camber Change
Camber is when a wheel is tilted so that the top of the tire is a different distance from the centerline of the car than the bottom of the tire. Negative camber is when the top of the tire is closer to the center of the car than the bottom of the tire. Positive camber is when the top of the tire is farther away from the center of the car than the bottom of the tire.

In circle track racing when turning left (in some countries cars turn right), we use positive camber on the LF wheel of the car and negative camber on the RF wheel. We can easily check the amount of camber by using a caster/camber gauge and reading the amount directly on the camber bubble vial or reading the digital readout for those types.

We have learned some interesting and important characteristics of tire camber for short track racing. We have always known that a racing tire will flex under the stress of cornering and the tread will move and roll under the wheel when the extreme forces associated with cornering are present. Different brands of tires have different stiffness of sidewall construction and therefore the amount of tire roll is different.

7/12From a driver’s view, the positive left front camber causes the top of that tire to lean out away from the centerline of the car. The opposite is true of the RF tire which has negative camber. On that wheel, the top of the tire leans in toward the centerline.

Tire temperatures tell us a lot about how much static camber we ultimately need so that the tire contact patch will be optimal at mid-turn. The overall goal is that we need the tire contact patch to be relatively flat on the racing surface at mid-turn in order for the tire to be able to provide the maximum amount of grip it's capable of giving. This is often referred to as the maximum "footprint."

Tire temperatures can alert us to improperly set static cambers. A front tire that is hotter on the inside edge (side toward the inside of the racetrack) usually has too much positive camber in the case of a LF wheel, or too much negative camber if it's the RF wheel.

The cambers will often change as the car dives and rolls as it enters and negotiates a turn. True camber change is a product of both chassis dive and chassis roll. Gone are the days when we would jack up the wheel and measure how many degrees the camber changed in each inch of bump. Those numbers really don't tell us anything.

Dive, or bump, is only part of the answer. Chassis roll has an effect that adds or subtracts from what dive does. So, what we really need to know is what the dynamic camber ends up at after the car dives and rolls, just like the chassis does in the turns.

8/12For measuring camber, just read the amount on the vial for a manual gauge or the display on a digital gauge. Make sure to calibrate your gauge before using. The Intercomp Caster and Camber gauges used in these photos are bubble type and digital. The caster and camber readings came out the same for both gauges. Make sure the car is at ride height with all of the weight in the car including the driver. Air the tires up to operating pressures. The setting for camber may need to change once the car is run hard and the tire temperatures have been evaluated at maximum operating levels.

The left front always loses a lot of camber, so we need to allow for that in setting the amount of static camber. Generally, if we end up with between 1/2 to 1 degree of positive camber at the LF wheel after the car dives and rolls, then that tire will have the dynamic camber that it needs.

The RF camber change is a little different. We can design our car so that the RF camber doesn't change after dive and roll for more conventional setups. This is actually exactly what that tire wants for most short track applications. The reason for this is that as we enter the turn, the RF tire takes a set fairly quickly. If the camber continues to change after that initial set, then the tire will give up traction and the car will usually push.

For some of the present day setups for both asphalt and now on dirt, teams are running softer springs that result in much more dive. This causes much more camber change associated with dive and we therefore need to make adjustments to our static camber settings to compensate.

9/12In order for your car to have near zero bumpsteer, two conditions need to exist. A line drawn through the centers of rotation of the ends of the tie rod needs to point at, and travel through, the instant center and the tie rod length needs to be the specific distance between the inner mounts and the ball joints as shown.

The right upper control arm angle mostly controls the RF camber change. So we try to work with that control arm angle first. Once we have the minimal camber change from dive and roll, we leave that angle alone as we check our frontend for moment center location and adjust the left upper arm angle as needed.

We can measure camber change by several different methods. In the shop, we can set the chassis ride heights just as they would be at mid-turn on the racetrack and then directly measure the camber at each wheel. To do this, we will need to know the shock travel at mid-turn, which is very hard to estimate.

If we look at the shock travel indicators on the shaft of the shock, they will always tell us total shock travel which includes braking, going over bumps, banking changes such as exiting the racetrack. So, an estimate based on how the car looks on the track could provide the best information as to the mid-turn attitude.

Bumpsteer
Once the caster and cambers have been set, it's now time to adjust our bumpsteer. As the front wheels move up and down, we want the front wheels to maintain a particular direction and not steer. It's most important for the wheels to have minimal bumpsteer when we are negotiating the turns. There are certain elements of the construction of the frontend components that will make this happen.

10/12If the tie rod is aligned pointing above the instant center, the wheel will bump out when moving up and bump in when moving down. The arc it follows will not be correct in relation to the arc of the ball joints.

The angles of the upper and lower control arms, meaning a line extending through the center of rotation of the ball joints and inner mounts of each arm, intersect at a point we call the instant center (IC). This is one of the components used to determine the moment center location. In order to have near zero bumpsteer, we need to have the tie rods on each side point toward the IC for its side. This is one of two criteria for near zero BS.

The other thing we need is for the tie rod to be a specific length. That length must be equal to the distance formed by 1) a line extending through the centers of rotation of the tie-rod ends, and 2) the tie-rod line intersection with a) lines extending through both the upper and lower ball joints, and b) the plane that passes through the inner chassis mounts. This can get a little complicated because although the ball joints do form a single line, the chassis mounts form a plane because of the position of the front and rear mounts.

So, the inner tie-rod intersection point is where the tie-rod line intersects the plane of the inner mounts and the outer line intersection point is where it intersects the ball joint line. A three dimensional geometry program can simulate this very well, but most of us don't own or know how to operate one of those or even have the patience to learn all of that. So, we must go through the process of physically measuring the BS in our cars.

11/12If the tie rod is aligned where it points below the instant center, then the wheel will bump in when traveling up and bump out when traveling down due to the difference in arc path to the ball joints.

When the tie rod isn't aligned with the IC and/or the length is wrong for the system, we have BS. As the wheel moves vertically, the wheel will either steer left or right. We will refer to the direction from a driver's perspective only in this discussion.

If the tie rod was pointed so the tie-rod line passes below the IC, then the wheel will bump in (toward the centerline of the car) as the wheel travels up and bump out when the wheel travels down. If the tie-rod line passes over the IC, then we will have bump out as the wheel travels up and bump in when the wheel travels down.

If the tie rod were too short, we would have bumpsteer in when the wheel travels in both directions from the static ride height position. If it were too long, then the wheel would bump out as the wheel traveled in both directions from ride height.

These indicators can tell us if we have either a tie-rod alignment problem or a tie-rod length problem. In some cases, both may be present and that causes a very erratic motion of the wheel. To determine which, record each inch of travel, for several inches, in both directions from static ride height and note the tendencies. You might have perfect alignment and a tie rod that is wrong for length. This could be due to a poorly designed drag link or the wrong width rack-and-pinion steering unit.

12/12With antidive, as the wheel moves up, the upper ball joint moves rearward. This causes the spindle to rotate and with it the steering arm. The outer tie-rod end will move up changing the front view tie rod angle. This changes the alignment with the instant center and will change the bumpsteer characteristics.

In both dirt and asphalt racing, anti- and pro-dive are used in various degrees. These effects cause changes to our BS. This is because with antidive, for example, when the wheel travels up, the upper ball joint moves toward the rear of the car and this rotates the spindle, from a right side view, counterclockwise. This rotation moves the outer tie rod end upward and changes the angle of the tie rod. Now, it no longer points toward the IC.

Where we had near zero BS before with no antidive, we now have BS when the right front wheel travels up. With pro-dive, we see a similar affect, the tie-rod end moves down with vertical travel and again the tie rod is misaligned with the IC.

If you originally checked your BS and found it acceptable and then experimented with antis, and didn't recheck your BS, you could, and probably do, have a problem, not statically, but dynamically in the mid-turn configuration.

When we steer our front wheels, we change the angles of our tie rods due to caster, camber, and degree of spindle on both sides. The tie-rod ends travel in an arc that isn't parallel to the ground. This changes the outer tie rod height and therefore the BS. It's for this reason that we recommend doing your BS with the wheels both straight ahead and then again with the wheels turned and bumped, equal to the mid-turn attitude for the track you will run.

What's Up Next?
In our next and final installment of this series, we'll examine toe settings and Ackermann, and rearend alignment and design for rear steer. We'll also touch on tire stagger and tire pressures as the final steps in completing our race car setup.